Fiber-based amplifiers have generated great interest recently due to their ability to amplify ultrafast pulses to energies comparable to conventional bulk solid-state systems while offering significant practical advantages, including compactness, reduction of complex components, and freedom from misalignment. The excellent heat dissipation of the fiber gain medium also offers greater long-term pulse stability. However, the smaller beam confinement and larger interaction lengths render them ˜106 times more sensitive to nonlinear effects then bulk solid-state amplifiers. To avoid nonlinear effects, the dominant of which is self-phase modulation (SPM), it is necessary to employ chirped-pulse amplification (CPA) where pulses are stretched before amplification in order to reduce peak power, and compressed afterwards. However, even with the largest practical stretched pulse durations of ˜1 ns, power must be scaled back so as to allow no more than 1 radian of nonlinear phase shift in the gain medium to prevent noticeable pulse distortion and broadening.
Compensation of SPM therefore shows great promise in helping fiber-based chirped-pulse amplification (CPA) systems achieve pulses with larger energy. Eliminating SPM can also remove some of the complications employed to avoid nonlinearities, such as the use of large core multimode fibers, photonic crystal fibers and the large amounts of dispersion necessary for stretching. Compensation of SPM in chirped-pulse amplification (CPA) systems has been shown by using the negative nonlinear index (n2) of some materials, using a spatial light modulator (SLM) in a pulse-shaping configuration, and by residual third-order dispersion (TOD). However, the wavelength dependence of semiconductor parameters degrades the quality of compensation for pulses less than ˜1 ps, and linear and two-photon absorption limit the thickness of the material and thus the amount of nonlinear phase that can be practically removed. The concerns in using SLM's are cost and complexity of compensation which requires nontrivial free space alignment as well as speed limitations of the SLM which is physically limited to less than ˜1 kHz. The use of residual third-order dispersion (TOD) is practical to implement, but only partially compensates SPM at best.
There is a need for improved methods and systems for compensation of self-phase modulation (SPM) in chirped-pulse amplification (CPA) systems.